How to to create own sales and inventory application???

Question

In python
https://www.sololearn.com/discuss/3097510/?ref=app

Answer 1

First you need to write down the specification of this app. What will be it's features, how will it work, how will it communicate with the user, how it saves data.
Typically then you would pick your tech stack. Most likely you will need a database. If this is supposed to be a web app, you'll probably want to use a framework such as Django.
Finally you can start coding. Write tests also, to make sure that any future changes to the code will pass the automated tests that are determined by the specification (see step 1).
If all is fine you can deploy the app in its final location, maybe in the cloud or some computer where it will run happily ever after.

Answer 2

https://www.sololearn.com/Course/CC-Mandatory-JUMP_LINK__&&__Python__&&__JUMP_LINK-for-Beginners/?ref=app

Begin a course (you've tagged python, so here is the course).
Learn the language.
Practise.
If you need help, search for answers.
If you still need help, ask for help.
Write your app.
If you need help, search for answers.
If you still need help, ask for help.

Voila!